The kinesins are a family of motor proteins that walk along microtubule fibers, powered by the hydrolysis of ATP. The functional units in vivo are dimers which appear to operate in concert much like a pair of feet. While kinesin family members all share a significant sequence similarity, surprisingly the family contains proteins which move in opposite directions along the microtubule. The goal of the current study is to reveal the structural basis for these proteins' amazing ability to transform stored chemical energy into directed motion.